Electronic components, for example, IC packages are fixed on a circuit board via solder and connected to a circuit formed on the circuit board via solder (hereinafter, a term “mount” will be used to mean “to fix” and “to electrically connect”). Solder mounting involves melting solder at a high temperature, which gives a thermal load to an electronic component and might damage the electronic component in the worst case. Particularly, lead-free solder, which is used lately, has a higher melting point and thus a higher probability of thermally damaging an electronic component.
Hence, various |mounting|[A1]methods for avoiding exposure of an electronic component to a high temperature have been proposed, which use an electrically conductive adhesive instead of solder.
Unexamined Japanese Patent Application KOKAI Publication No. 2001-189337 discloses an electronic circuit device, in which an IC package is mounted on the electrode pads on a circuit board by transferring an electrically conductive adhesive to electrode bumps, where the electrode bumps are projection portions formed on the terminal electrodes of the IC package. Each electrode bump is formed of a lower bump to be fixed to the terminal electrode and an upper bump stacked on the lower bump. The upper bump has a compressive elastic modulus smaller than that of the lower bump. This allows the upper bump to compressively deform to absorb any unevenness of the circuit board in its flatness or any unevenness among electrode pads in their thickness, keeping the IC package in fine connection.
However, because of the fluidity of the electrically conductive adhesive before it has hardened, the electrically conductive adhesive that exists between the IC package and the circuit board under the pressure applied by the IC package with its own weight, spreads and comes into contact with adjoining electrode pads, risking formation of a short circuit.
Hence, Japanese Patent Publication No. 3549017 discloses an electronic circuit device, in which each of the electrode pads on a circuit board has a recess. The method of this electronic circuit device can prevent an electrically conductive adhesive from spreading over the circuit board, because the electrically conductive adhesive will be trapped in the recesses of the electrode pads.
In this case, however, there still remains difficulty with appropriately controlling the volume of the electrically conductive adhesive to be transferred to the electrode bumps; if the volume of the electrically conductive adhesive transferred to the electrode bumps is inappropriate, even the method of the electronic circuit device described above will cause the electrically conductive adhesive to overflow from the electrode pads.
Unexamined Japanese Patent Application KOKAI Publication No. 2006-32632 discloses an electronic circuit device, in which an IC package has a recess on the circumference of each of its electrode bumps, while a circuit board is covered with a resin sheet, from which an electrically conductive adhesive partially protrudes.
Admittedly, the method of the electronic circuit device of Unexamined Japanese Patent Application KOKAI Publication No. 2006-32632 can securely prevent an overflow of the electrically conductive adhesive, because the electrically conductive adhesive will be trapped in the recesses formed on the circumference of the electrode bumps of the IC package. However, this method is not practical because it takes complication in manufacturing the IC package and the circuit board.